Beginners Guide to ISE Measurement. Chapter 12.

TYPES OF MEASURING DEVICES AND DATA PROCESSING

The earliest type of pH meters were simple analogue devices with a resolution of only one or two millivolts. The original meters were calibrated in millivolts and the corresponding pH value was read from a calibration graph as described earlier. However it was soon realised that pH electrodes are reasonably uniform and reproducible instruments with very similar slopes so that it is not necessary to have a unique calibration graph for each electrode. In this case the meters can be calibrated directly in pH units by the manufacturer and can simply be recalibrated each time they are used (to compensate for temperature changes or slight differences in electrode response) by immersing the electrode in just one pH buffer solution and adjusting the meter output to give the correct reading. This type of meter is very simple and quick to use and is perfectly adequate for many pH measurements because it requires a change of more than 5 mV in order to change the pH value by more than 0.1 pH units.

Nevertheless, as noted earlier, in the direct measurement of ionic concentration, a change of only 1 mV will represent a change of 4% in the concentration of mono-valent ions and 8% for di-valent ions. Therefore it is necessary to have a more precise measuring system in order to determine ionic concentrations and for the most precise pH measurements. Furthermore, Ion Selective electrodes are more variable in their response and do not have such uniform slopes so it is more important to make a complete multi-point calibration in order to determine the slope for each electrode, each time it is used. Thus simple pre-calibrated analogue meters are not appropriate for ISE measurements and in this case it was always necessary for the analyst to keep a record of the millivolt readings obtained in the standard and sample solutions. Then a calibration graph was plotted on semi-log paper, using concentration units on the logarithmic axis, and the sample results were read directly from this graph, with varying degrees of accuracy depending on the range of concentrations plotted and the skill of the operator.

A major advance was made when digital meters were introduced with a resolution of 0.1 or even 0.01 mV. This enabled the analyst to measure and read the voltage with much greater accuracy and meant that the stability and reproducibility of the electrode response became the main limiting factor in determining the accuracy and precision. Nevertheless, although these meters were also capable of being calibrated in pH units, it was still necessary for the analyst to keep a log of the millivolt readings for ISE measurements and then plot the calibration on graph paper or using computer graphics. Sample results were then read from this graph, or calculated manually or by using computer spreadsheets, as with analogue meters.

The next major advance was when micro-processors were introduced which contained simple programs to calculate the slope and intercept from the calibration data and then use these to calculate the sample concentration from the millivolt reading in the sample. Thus the analyst can simply enter the concentrations of the standards and measure the millivolts, then immerse the electrodes in the sample and read the sample concentration directly from the meter. Nevertheless, these meters are often quite confusing to operate, with small keypads and multi-function switches, and they are not suitable for working in the non-linear range of the electrodes, or using different slopes for different parts of the calibration range, or for measuring more than one ion at a time. Furthermore, it is often difficult for the analyst to assess the quality of the calibration or detect errors in data entry and it is still necessary for the results to be transferred manually to a permanent record.

Most of these problems of data acquisition, transfer, processing, and storage have now been overcome with the introduction of meterless computer interfaces and sophisticated software packages which are the subject of the next section of this guide.